Contact lenses may be classified in various ways. If classified by number of optical powers they are usually divided into single vision, bifocal, or multifocal lenses. Single vision lenses may be comprised of spherical, aspherical and toric surfaces. Bifocal and multifocal contact lenses can also have spherical, aspherical and toric surfaces. Bifocal contact lenses are lenses with at least two regions of different focal length optical powers, known as zones or segments. Usually, one power is chosen to provide the wearer with clear distance vision and the second power to provide clear near vision, but any two powers may be selected. Multifocal contact lenses are usually comprised of at least three different optical powers or regions of variable power, as in U.S. Pat. No. 4,693,572 (Tsuitaki), U.S. Pat. No. 5,517,260 (Glady) and U.S. Pat. No. 5,754,270 (Rehse).
Multifocal and bifocal contact lenses generally are classified into two types, concentric and vertically segmented. Both types can be produced as rigid or soft contact lenses. Concentric power lenses can be further characterized as having concentric zones of two or several constant spherical powers, with or without aspheric intermediate zones. Concentric designs refract light through each of the zones simultaneously. As a result, multiple images are projected onto the retina creating a circle of confusion or “blur circle” which is transmitted to the brain. The ability of patients to resolve the full range of near, intermediate, and distance images from the blur circle varies greatly among patients. The ability to resolve simultaneous images is further impaired as the patients become older with less crystalline lens accommodative capability, and continued reduction of pupil size.
Vertically segmented lenses have vertically separated power zones, an upper zone that usually provides the appropriate correction for viewing far distances and a lower zone, which usually provides the appropriate correction for viewing near distances, and if present, a middle zone for intermediate distance viewing. The lenses are designed to alternate their position in front of the pupil when the lens moves up and down on the eye as the result of lid forces, which occur when the wearer changes gaze between different distances, a process called alternating vision, as described in U.S. Pat. No. 3,597,055 (Neefe) and U.S. Pat. No. 3,684,357 (Tsuetaki). Because this type produces single power images at the visual axis, the lenses are effective for those brains which do not resolve multiple images, and for those patients who require high near power correction. However, these lenses have proven inadequate for clear vision among patients requiring acuity at a wide array of constantly varying intermediate focal lengths such as for computers, cell phones, and other such purposes.
The vertically separated power zones maintain their relative positions by various features that can be added to control the lens position and stabilize the meridional rotation as described in U.S. Pat. No. 4,095,878 (Fanti); U.S. Pat. No. 4,268,133 (Fischer); U.S. Pat. No. 5,760,870 (Payor); U.S. Pat. No. 5,296,880 (Webb); and U.S. Pat. No. 4,573,775 (Bayshore). This is commonly accomplished in rigid bifocal contact lenses by incorporating differences in thickness of the lens in any manner which abuts the upper and/or lower lid. The lid will tend to squeeze or push a thicker portion of the lens away. Common methods of altering thickness include thin zones at the top or bottom of the lens, thick “bumps” strategically placed to bump into the lids, or prism, to create this function. Prism is well suited to a translating lens to maintain the desired lens orientation and keep the lower zone of the lens downward on the eye as described in U.S. Pat. No. 5,430,504 (Muckenhirn) and U.S. Pat. No. 4,854,089 (Morales) and in Burris, 1993; Bierly, 1995, and Conklin Jr. et al, 1992. The lower edge of the lens is designed to rest upon the lower lid margin of the wearer and the lens shifts up and down relative to the eye as the result of lid forces. There are several subtypes of vertically segmented multifocal contact lenses, based on the shape of the near zone, including round, D-shaped, flat, crescent, and others as described by Conklin Jr. et al, 1992 and in U.S. Pat. No. 4,618,229 (Jacobstein) and U.S. Pat. No. 5,074,082 (Cappelli).
U.S. Pat. No. 6,746,118 to Mandell describes a contact lens comprising a secondary prism that controls vertical lens movement on the eye of a wearer. The anterior surface of the lens has a central optical portion, which in one embodiment contains a bifocal design comprising a distance zone located above a near zone. The secondary prism has a base that extends forward from the lower region of the anterior surface of the lens. When the lens is worn, the base is in apposition or near apposition to the lower lid so that as the wearer looks downward the lid holds the lens in place, which produces an upward movement of the lens relative to the eye. This allows the wearer to view through the lower part of the central optical portion, which contains the optical power for near vision.
U.S. Pat. No. 6,746,118 to Mandell also describes a method for manufacturing a contact lens with secondary prism, which involves a process whereby a lens button, consisting of a cylinder of contact lens material, is machined in a series of steps using an optical lathe. In machining the front surface the first step is to form the button into a shape resembling the top of a hat. Next, the peripheral portion of the hat is shaped to form in part a primary prism and then the central portion of the hat is shaped to form in part a secondary prism and the power zone(s). Various other lens features are added for design enhancements.
U.S. Pat. No. 6,871,953 to Mandell, the disclosure of which is incorporated by reference in its entirety, describes a monocentric bifocal contact lens with upper and lower optical power zones is connected by a transition comprising a family of sigmoidal curves. The sigmoidal curve begins with a common tangent at the boundary of the near zone and, with a reversal of sign from the near zone curve, climbs with increasing positive slope to an inflection point, whereupon it continues to climb with decreasing positive slope until reaching the distance zone curve, with which it has a common tangent. A sigmoidal curve can be constructed from numerous mathematical functions, examples of which include polynomial, conic, transcendental, or cumulative distribution curves.